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> A "back Yard" Experiment, Fridge, Run On Inverter, raw data
pianotuna
post Apr 24 2010, 09:58 PM
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Hi all,

This is the raw data from a "back yard" experiment. I wanted to know how long the Dometic Royale" Tri-Fuel Fridge (6 cu. ft.) would take to start to cycle, and how much electric energy would be used to reach that point. I further wanted to know how much energy, including cool down time, would be used by the fridge in 24 hours. I feel that may give me a "worst case" scenario for travelling and running on 110 volt via the inverter. The fridge and freezer are empty except for a thermometer. I know already that the fridge runs at about 4 C, when it is running. I don't have that information for the freezer yet, so I decided to monitor the temperature in the freezer.

I'm lucky enough to have 256 watts of Uni-Solar panels, with a Blue Sky 3024di MPPT temperature compensated charge controller. The panels are wired in series/parallel to reduce line losses. The system runs at 33 volts. It will be interesting to see how much energy they return to the battery bank. Saturday's forecast is for clear weather. The battery banks (875 amp hours of 12 volt Walmart deep cycle) have been at float state of charge for one week--the charging amps in peak sun show 0.0 amps. The inverter is a Cobra 2550 modified sine wave (2500 watt, 5000 watt surge) and has an overhead of 0.5 amps.

I connected my kill-o-watt meter between the output of the inverter and the shore power cord, so I will have a running total of the watts used.

19:45 (7:45 pm) parasitic loads only for five hours; volts reading 13.4 charging at 0.1 amp. The out door temperate is 8 C; temperature inside RV 11.7 C; temperature in freezer 11.7 C

I turned on inverter and the volts reading dropped to 13.1 after about five minutes. The AC voltage was 113 volts

I turned on the Fridge on the 110 volt setting (298 watts as registered by kill-o-watt, or ~30 amps load on the battery banks) and the voltage dropped to 12.9; kilowatts 0.0; and AC voltage increased to 116 volts.

20:52 volts 12.6; out door temperature 5 C; temperature inside RV 10.5 C; temperature in freezer 0 C; kilowatts 0.34 (running total = rt)

21:48 volts 12.5; out door temperature 3 C; temperature inside RV 9.9 C; temperature in freezer -15 C; kilowatts 0.62 (rt)

22:47 volts 12.5-4 (bouncing); out door temperature 2 C; temperature inside RV 9 C; temperature in freezer -18 C; kilowatts 0.75 (rt)

23:45 volts 12.7; out door temperature 2 C; temperature inside RV 8 C; temperature in freezer -19 C; kilowatts 0.92 (rt)

6:45am volts 12.5; out door temperature 0.9 C; temperature inside RV 2.8 C; temperature in freezer -7C; kilowatts 1.38 (rt); charging at 0.2 of an amp

7:45 volts 12.3-2 (bouncing); out door temperature 3 c: temperature inside RV 3.2 C; temperature in freezer -7 C; kilowatts 1.44 (rt); charging at 1.5 amps; cooling fan on inverter was running

8:45 volts 12.5; out door temperature 7 C; temperature inside RV 4.4 C; temperature in freezer -12; kilowatts 1.66 (rt); charging at 1.2 amps (panels partly shaded)

9:45 volts 12.7; out door temperature 10 C; temperature inside RV 5.5 C; temperature in freezer -10; kilowatts 1.66 (rt); charging at 2.5 amps (panels partly shaded)

10:45 volts 12.5; out door temperature 14 C; temperature inside RV 6.5 C; temperature in freezer -7; kilowatts 1.88 (rt); charging at 3.9 amps (panels partly shaded)

11:45 volts 13.3; out door temperature 17 C; temperature inside RV 8.7 C; temperature in freezer -13; kilowatts 1.91 (rt); charging at 11.1 amps

13:00 volts 12.7; out door temperature 17 C: temperature inside Rv 13.1; temperature in freezer -16; kilowatts 2.14; charging at 4.0 amps (mixed cloud); in full sun voltage 13.2 and charging at 15.5 amps

13:50 volts 12.3-2 (bouncing); out door temperature 17 C; temperature inside RV 14.7; temperature in freezer -11; kilowatts 2.26; charging at 4.3 amps (cloud cover)

14:50 volts 12.6; out door temperature 18 C; temperature inside RV 16.7; temperature in freezer -16; kilowatts 2.39; charging at 10.2 amps; in clouds voltage 12.2 and charging at 3.1 amps.

15:50 volts 12.6; out door temperature 17 C; temperature inside RV 18.8; temperature in freezer -18; kilowatts 2.56; charging at 3.7 amps overcast thin clouds

16:45 volts 12..7; out door temperature 18.2 C; temperature inside RV 20.6; temperature in freezer -16; kilowatts 2.76 charging at in variable clouds at 8.1 amps

17:45 volts 12.6; out door temperature 18.2 C; temperature inside RV 20.8 C; temperature in freezer -16; kilowatts 2.96; charging in variable clouds at 5.1 amps

18:45 volts 12.0 ; out door temperature 15 C; temperature inside RV 21 C; temperature in freezer -16 C; kilowatts 3.14; charging at 1.3 amps

19:45 volts 11.9; out door temperature 12 C; temperature inside RV 19.9 C; temperature in freezer -16 C; kilowatts 3.30; no charging.

I'm open to other folks interpretations on the raw data.


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Don
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Denali
post Apr 25 2010, 09:02 AM
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Since your voltage started out at 13.1-13.4 and ended up at 11.9, that's consistent with your 298 watts of consumption exceeding the maximum 256 watts of solar power available, no?


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Dave Rudisill
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pianotuna
post Apr 26 2010, 03:16 PM
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Hi Denali,

There was very little charging going on for most of the 24 hours. I should pray to the sun gods a bit better?


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Don
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Denali
post Apr 27 2010, 08:38 AM
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QUOTE(pianotuna @ Apr 26 2010, 02:16 PM) *

Hi Denali,

There was very little charging going on for most of the 24 hours. I should pray to the sun gods a bit better?
That might work.

Or...continually drive east at 1,041 MPH to keep the sun overhead.


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Dave Rudisill
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pianotuna
post Apr 27 2010, 11:43 PM
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Hi Denali,

I was not aware that the sun rose in the West and set in the East *grin*. I think perhaps I'll try driving west at 1676 kilometers per hour? I wonder how many watts that would take to do LOL!


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Don
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Denali
post Apr 29 2010, 11:33 AM
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QUOTE(pianotuna @ Apr 27 2010, 10:43 PM) *

Hi Denali,

I was not aware that the sun rose in the West and set in the East *grin*. I think perhaps I'll try driving west at 1676 kilometers per hour? I wonder how many watts that would take to do LOL!
It's a good thing I have a GPS to guide me. I'd get lost in a paper bag.


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pianotuna
post Apr 29 2010, 11:57 PM
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Hi all,

I wanted to know how long the fridge took to cycle starting from ambient temperature (2 to 3 hours).

I wanted to know how much energy was needed to do that (75 amp-hours).

I wanted to know how much energy was used in a 24 hour period (330 amp-hours).

I wanted to know how much temperature variation there was in the freezer after the unit cycles (-7C to -19 C but after 20 hours it appears to swing less).

Propane is not easily available in many areas where I boondock, and used to be "extra pricey" in Canada. There is not nearly the price difference anymore between USA and Canada.

The fridge does not perform as well on the 12 volt setting unless battery voltage is 13.9--then it seems to be just as good on 12 volt as on 110 volt.

Some ideas suggested by the data:

a. 3.3 kilowatts or 330 amp-hours; from that, divide by five and conclude that 660 watts of solar panels may be needed to run a three way fridge, worst case.

b. also from that 3.3 kilowatts suggests that 660 amp-hours of storage batteries are needed.

c. the fridge appears to use about 20 amps per hour, and it does cycle. That means it may safely run the engine alternator via the inverter.

d. run time may be approximately 40 minutes per hour, or a 2/3 duty cycle.


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Don
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Trentheim
post Apr 30 2010, 08:09 AM
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Don,

As you probably know but some of the readers of this thread might not be aware of is that the initial energy requirements from ambient temperature to operating temperature will be your biggest. Which means that you can get a head start if you put cold things in your fridge and freezer before you start it up (such as, say, ice).

Once your unit reaches operating temperature (equilibrium), the energy load is "minimal" and only needs to really use any electric to maintain the desired temperature "every so often" (like the furnace or AC in your house).

I think it would be a good follow-up to your experiment to run the unit at equilibrium for a while and observe the required energy requirements. Maybe even follow up with that by a time of "normal use" by opening and closing the unit at breakfast, lunch, and dinner times to see what the panels can do.

Love the experiments, keep 'em up.

JT



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pianotuna
post Apr 30 2010, 05:06 PM
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Hi JT,

That is what I thought too--but I was dead wrong. Once the unit did read equilibrium it drew about 200 watts per hour--giving an amazingly high duty cycle of 2/3--I was hoping for 1/6 or less.

I do agree I need to try another test--this time with freezer packs to simulate the fridge being full of food. That testing can be done on shore power--and I'll probably do so some time.

Someone kindly produced a spreadsheet from the results. When ambient temperature was high the freezer stopped "bouncing" and stayed rather cold at about -16 C. That makes sense as there is no sensor in the freezer--it relies on the system going through the freezer first--and then to the fridge. No demand in the fridge = warmer temperatures in the freezer.

How about doing some testing yourself and posting?

QUOTE(Trentheim @ Apr 30 2010, 08:09 AM) *

Don,

As you probably know but some of the readers of this thread might not be aware of is that the initial energy requirements from ambient temperature to operating temperature will be your biggest. Which means that you can get a head start if you put cold things in your fridge and freezer before you start it up (such as, say, ice).

Once your unit reaches operating temperature (equilibrium), the energy load is "minimal" and only needs to really use any electric to maintain the desired temperature "every so often" (like the furnace or AC in your house).

I think it would be a good follow-up to your experiment to run the unit at equilibrium for a while and observe the required energy requirements. Maybe even follow up with that by a time of "normal use" by opening and closing the unit at breakfast, lunch, and dinner times to see what the panels can do.

Love the experiments, keep 'em up.

JT



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Don
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John S.
post May 9 2010, 11:33 AM
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Ok, I thank you for the results. I have done this too on my Foretravel by switching the plug in the back to be powered by the inverter. It handled the load but I had 4 8D Gell Batteries. I asked the engineer who designs the systems for FOretravel what would be the issue running it on the inverter. He said they use bigger inverters and or two of them on electric coaches. They have larger alternators and it reduces the battery life tremendously. the number I was given is about half the life vs normal. It is more efficent on propane too. I would also say you might need to run the generator a bit and that is easier and more efficient to keep up the charge. You might also increase the number of panels you have up there too. I had 3 110 watt panels on my roof. SO, running down the road with the alternator and the inverter could keep up fine it was sitting still that the problem came about. If I was to go all electric they will almost triple the battery amp hours.


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pianotuna
post May 9 2010, 07:00 PM
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Hi John,

That was interesting to hear from the engineer.

I don't have a built in generator.

I've not run my generator except to exercise it for a year now.

I last bought propane in May of 2009. (I'm a propane miser, clearly LOL)


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Don
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